It is estimated that fish disease cost twenty to thirty cents for each dollar spent rearing fish in the USA. Although fish pathogens include fungal, protozoan and bacterial agents, it is viral diseases that most concern hatchery owners, as they are largely uncontrollable. In fact, there are no effective antibiotics or other antiviral agents that work effectively against viruses in fish.
Massive mortality of Cyprinus carpio species has been observed in food and ornamental trade fish farms in many countries, resulting in severe financial losses. Although the lethal disease is highly contagious and extremely virulent, morbidity and mortality are restricted to Koi and Common carp populations. Several closely related species, including other Cyprinoids such as Goldfish, were found to be completely asymptomatic to the disease, even following long-term cohabitation with diseased fish sharing the same tank.
The intensive farming of Koi, Common carp and other Cyprinoids in ponds or in captivity results in frequent distribution of viral diseases in these populations. Corona-like virus (Miyazaki, 2000), rhabdovirus (Kim, 1999) iridovirus (Shchelkunov, 1990) and herpesviruses (Sano, 1985; Hedrick, 1990, 2000; Calle, 1999) have been suggested as the cause for the severe diseases in Cyprinoids. Herpesvirus was detected in papillomatous skin growth of Koi carp in North America (Hedrick, 1990; Calle, 1999). This carp herpesvirus (CHV) is consistent with herpesvirus cyprini known in Koi carp populations in Japan (Sano, 1985, 1991). A lethal disease observed in Cyprinoids in Israel has also been observed in North America, Europe and Korea (Hedrick, 2000; Walster, 1999; Oh, 2001).
It has been suggested that the disease causing massive mortality of Cyprinus carpio species is caused by a Koi herpesvirus (KHV). However, in actuality the KHV virus has only partly been characterized (Hedrick, 2000; Gray, 2002; Body, 2000). Irrespective of the identity of the virus, the disease exhibits a distinct development pattern. Affected fish exhibit sluggish behavior, followed by death. In the period that precedes death, white patches appear on the gills. These are produced by necrotic gill tissue and extensive mucus production and may be accompanied by bleeding (Dawes, 2002).
Currently, there are no methods for controlling the disease except for the destruction of infected stocks and the decontamination of hatchery facilities. Although mortality can reach as high as 100%, some fish can and do survive and at times the survival rate exceeds 20%. These fish then become resistant to subsequent exposures to the virus, remaining healthy despite attempts to reinfect them.
On the basis of these and other related observations, local farmers in Israel established a seven-step protocol designed at creating naturally immune fish that might be considered safe, clean and suitable for sale (Dawes, 2002).
This seven-step protocol involves allowing fish to spawn and hatch in March and grow unsorted until July, at which point they are sorted into different quality categories. The sorted fish are then exposed to the virus for four days through the introduction of sick fish into the tank and are subsequently given two three-month recovery periods. The fish are-then allowed to experience the optimal infection window that occurs as the temperatures drop at the beginning of October and are subsequently tested for immunity around January.